Sample Size Calculation Metabolic control, baseline retinopathy and diabetes duration are the only independent predictors of progression to PDR (Porta 2001, Skrivarhaug 2006). A recent study examining the effect of protein kinase C inhibition on the progression of DR enrolled subjects a similar spectrum of DR severity as is planned for our study (PKC-DRS 2005). The progression rate over 3 years in the study was 50%, lower than their estimated 64% (ETDRS 1991c). We will use the more conservative estimate of 50% for our sample size calculation.